1. Technical Field
The present invention relates to a power-supply noise measuring circuit and a power-supply noise measuring method.
2. Background Art
In recent years, as the operation speed of a large-scale integration (LSI) circuit increases and the power consumption of the LSI circuit decreases, power-supply noise is undesirably generated due to power-supply voltage fluctuation, for example. The power-supply noise has a great influence on a delay design of the LSI circuit. As the power-supply noise increases, a delay variation increases, with the result that it is necessary to widen a margin for a timing design.
The margin widened by as much as the delay variation relative to a timing window that is reduced following acceleration of signal transmission within the LSI greatly influences realization of the signal transmission. It is, therefore important how accurately the amount of power-supply noise can be grasped.
It is necessary to measure the power-supply noise at any time in years to come. Therefore, a power-supply noise measuring circuit that is small enough to be mounted on any LSI circuit and has enough accuracy is needed.
Japanese Unexamined Patent Application Publication No. 2005-249408 discloses a power-supply noise measuring circuit. In this technique, as shown in FIG. 8, power-supply noise is measured by comparing the power-supply noise with a reference voltage using a comparator circuit 106.
In addition, as shown in FIG. 9, Intel Corporation discloses a power-supply noise measuring circuit in Publication “2003 Symposium on VLSI Circuits Digest of Technical Papers, 4-89114-035-6/03, FIG. 5”. In this technique, power-supply noise is measured by extracting a minute change of a reference power-supply and a measuring power-supply, and by comparing the power-supply noise with a reference voltage.
As another example, as shown in FIG. 10, a power-supply noise measuring method using a sampling oscilloscope circuit is disclosed in Publication “ISSCC 2002/Feb. 5, 2002/Salon 10˜15/9:00 AM, FIG. 11.2.1”. In this technique, data is acquired at a data sampling timing that is shifted by one clock period with respect to a periodically-varying signal using, principles of the sampling oscilloscope. Therefore, the data acquiring period is increased T/ΔT times (T is one clock period, ΔT is resolution performance of data acquisition), and the data is retrieved as a low speed output.
However, in any of the techniques described above, it is necessary to mount a large amount of decoupling capacitors so as to prevent a measuring circuit itself from being affected by the power-supply noise. It is also necessary to mount a filter circuit in the power supply to perform denoising.
This causes a problem that the circuit area increases. Further, when the power-supply noise measuring circuit is mounted in the LSI circuit, an area I′m mounting the power-supply noise measuring circuit needs to be prepared in advance. However, an increase in the area where the power-supply noise measuring circuit is mounted greatly influences a floorplan.
Moreover, it is preferable, to dispose the power-supply noise measuring circuit near a logic circuit with high availability ratio. However, the increases in the area where the power-supply noise measuring circuit is mounted causes a problem that the power-supply noise measuring circuit is not always arranged at a desired area.